1. Field of the Invention
The present invention relates to reservoir simulation by computer processing and more particularly to processing input data from a subsurface reservoir in history matching.
2. Description of the Related Art
In the oil and gas industries, the development of underground hydrocarbon reservoirs often requires the building of computer simulation models. These underground hydrocarbon reservoirs are often complex rock formations which contain both a petroleum fluid mixture and water, with the reservoir fluid content, often existing in two or more fluid phases. The petroleum mixture is produced by wells drilled into and completed in these rock formations.
Reservoir simulation belongs to the general domain of simulation of flow in porous media. However, reservoir simulation normally involves multiple hydrocarbon components and multiple fluid phases in an underground geological formation which is under high pressure and temperature. The chemical phase behavior of these hydrocarbon fluids and the included groundwater has to be taken into account in these simulators.
Sometimes, fluids such as water and/or gases are also injected into these rock formations to improve the recovery of the petroleum fluids. Simulation models therefore contain data which describe several types of information: the specific geometry of the rock formations and the wells, the fluid and rock property data, as well as production and injection history pertaining to the specific reservoirs of the oil or gas field in question.
Oil and gas companies have come to depend on reservoir simulation as an important tool to enhance the ability to exploit a petroleum reserve. Simulation models of reservoirs and oil/gas fields have become increasingly large and complex. The simulator (known in the petroleum industry as a reservoir simulator) which in the past has run these models was a computer operating under the control of a set of computer instructions or software. The software was in coded form, including some specific numerical algorithms and data constructs of an underlying mathematical model. The mathematical model which represented the physics of fluid movements in these hydrocarbon reservoirs was a system of nonlinear partial differential equations which described the transient multiple-phase, multiple-component fluid flow and material balance behaviors in these reservoirs. The fluid flow and material balance changes were induced by the production and/or injection of fluids as well as the pressure-volume-temperature (PVT) relationships of the reservoir fluids.
The reservoir simulator simulated the material balance and fluid flow in underground reservoirs and the included surrounding porous rock formations by subdividing the volume into contiguous cells known as grid blocks. A grid block was the basic finite volume where the mathematical model was applied. The number of grid blocks needed varied depending on the resolution needed for the simulation and the size of the reservoirs and oil and gas fields in question.
An example of a simulator is one known as GigaPowers, which is a parallel oil, water and gas enhanced reservoir simulator for what are known in the industry as giant reservoirs. The GigaPowers simulator solves highly complicated mathematical equations about reservoir parameters based on the physics, chemistry and thermodynamic relation of reservoir structures and their fluid contents. The reservoir is organized into a number of cells or grid blocks. For a giant reservoir, the number of cells can be at least a million or more, and can even reach or exceed a billion, depending on the desired degree of resolution in the simulation model. The initial input data to this reservoir simulator is the state of the reservoir at the beginning of the simulation in addition to injection and production rates of various fluid during time. From this data the simulator computes the state of the reservoir during a sequence of time intervals. The input to the simulator is known as the model.
Periodically it is necessary to perform a procedure known as “history matching” of the model. History matching is a process by which historical data obtained from geologic studies, from interpretation of data obtained from drilled wells, and from production and injection rate measurements in the field are compared with parameter values produced by the simulator. The historical data is provided as inputs to the reservoir simulator. The simulator is then run with the historical data and the output from the simulator is compared with field measurements. If there is a significant discrepancy between simulation results and measured data in the field, the input data to the simulator is adjusted until a reasonable match is obtained. This is known as a modification of the input data.
A history match of an existing oil field that is in production thus initially involves running a computer program called a reservoir simulator with historical data provided by geoscientists and engineers to the simulation personnel. The output from the reservoir simulator, which represents fluid pressures in the field and rates at the different wells, is then compared to the well rates and field pressures measured and recorded during the production history of the reservoir. If there is close agreement between the results of the simulator and the field recorded data, then it is said that the reservoir has been history matched and the current input data, called the model, can be used to predict the behavior of the reservoir in the near future. On the other hand, if the results from the simulator disagree with the values measured in the field, the input data to the simulator needs to be adjusted. This adjustment should ideally be based on the original data with minor modifications. Often considerable time and effort by several different staff members was involved in history matching. Those involved, who are typically reservoir engineers, in history matching needed to modify the model data in order to improve the history match. A full cycle of modifications of the model was required. These modifications could be very sophisticated in nature and the best way to represent them was as algebraic and logical manipulations of the original data.
For a history match of an oil field many simulation runs with different input data were needed. So far as is known, the input data has been changed by modifying previous data in some fashion and rerunning the simulation.